Bring a kettle to the boil. Pour the water over the dried porcini just to cover.
Heat half the butter in a saucepan. Gently cook and stir the onion, garlic and thyme for 5 minutes until softened and starting to brown.
Drain the porcini, reserving the juice. Add to the onion with the mixed wild mushrooms. Leave to cook for 5 minutes until they go limp.
Pour over the stock and the reserved mushroom juices. Bring to the boil, then simmer for 20 minutes.
Heat the remaining butter in a frying pan. Fry the bread cubes until golden. Drain on kitchen paper.
Stir in crème fraîche, then simmer for a few more minutes. Blitz the soup with a hand blender or liquidiser. Pass through a fine sieve, then serve with the croutons, chopped chives and a drizzle of truffle oil.